Various petroleum extraction subterranean formation treatment procedures require use of compositions having high viscosities, such as during drilling or stimulation treatments. Various viscosifiers are used with clay in order to achieve a desired viscosity or degree of fluid loss control. Further, various viscosifiers are used with non-water-soluble weighting agents such as barite to provide a desired density. However, the use of clay and non-water-soluble weighting agents can cause severe formation damage due to plugging of the pores of the reservoir formation and due to difficulty of clean-up (e.g., clays are not water soluble).
Drill-in fluids, or reservoir drilling fluids, are a special type of drilling fluids used when drilling in the reservoir section of a subterranean formation. These fluids generally include base fluids (such as brine), acid-soluble bridging agents, water-soluble polymers, pH stabilizers, and oxygen scavengers. Clays and non-water soluble weighting agents are generally avoided in drill-in fluids to prevent pore plugging and to avoid difficult clean up. Instead, heavy brines are generally used to provide the density, and water-soluble or water-swellable polymers are used as the rheology modifiers and fluid loss control agents. These polymers are generally biopolymers with a temperature limit below 300° F. At reservoir temperatures higher than 300° F., synthetic water-soluble polymers can be used. However, many viscosifiers cannot provide adequate viscosity at high temperatures, especially over time. In addition, many viscosifiers are not as efficient when used in brines. Thermal breakdown of polymers and high salt content of the heavy brine used continue to pose a very challenging hurdle for high temperature applications.
Synthetic polymers for temperature control and fluid loss control at high temperatures are generally linear or lightly crosslinked (e.g., less than 1 mol % crosslinker) and need to be used with clay to achieve desired viscosity and fluid loss control. Without clay, unfortunately, these polymers cannot provide desired viscosity and fluid loss control needed for various subterranean treatments.